1. Field of the Invention
The present invention relates to an electric power steering device that uses an electric motor as a source for generating a steering assistance force.
2. Description of Related Art
As electric power steering devices for vehicles, a device having the configuration disclosed in Japanese Patent Application Laid-Open No. 2002-21943, for example, is general. This electric power steering device is configured to detect the steering torque applied to the input shaft depending on the relative angular displacement amount between the input shaft connected to the steering wheel and the output shaft coaxially connected to the input shaft via a torsion bar and to drive a steering-assisting electric motor on the basis of the detected torque. In this kind of electric power steering device, the rotation force of the electric motor is transmitted to a steering mechanism via a reduction gear mechanism, and the operation of the steering mechanism depending on the rotation of the steering wheel is assisted, whereby the driver's load for steering is reduced.
The reduction gear mechanism comprises a worm serving as a small gear and connected to the driving shaft of the electric motor and a worm gear serving as a large gear and meshing with the worm, and this worm wheel is fitted on and secured to the intermediate portion of the output shaft.
In addition, the worm has shaft portions protruding from both end portions of the tooth portion (gear body) thereof, and the shaft portions are rotatably inserted into the inner rings of roller bearings so as to be born, respectively.
Conventionally, the worm of the electric power steering device configured as described above is born so that the shaft portions at both end portions cannot move in the axial direction with respect to the roller bearings being used respectively to bear the shaft portions. Hence, in the case where the device is configured that, when the steering wheel is steered left or right from the steering neutral position, steering assistance is carried out by the rotation of the electric motor in the early stage of the start of the steering, the steering assistance is carried out even if the steering angle of a vehicle running at high speed is a small angle of approximately one degree, for example; hence, steering feeling is deteriorated. Hence, conventional electric power steering devices are generally configured that the electric motor is not driven when the steering angle is a small angle of approximately one degree, and that the electric motor is driven only when the steering angle exceeds an appropriate steering angle.
In the conventional electric power steering devices configured that the electric motor is not driven until the steering angle exceeds the appropriate steering angle as described above, when the steering is in a range wherein the electric motor is not driven, in other words, when the steering is performed in the vicinity of the steering neutral position, the steering force of the steering wheel is transmitted to the driving shaft of the electric motor via the input shaft, the torsion bar, the output shaft, the worm wheel and the worm, whereby the driving shaft is rotated. As a result, the load for rotating the driving shaft of the electric motor is applied reversely to the steering wheel via the worm, the worm wheel, the output shaft, the torsion bar and the input shaft; hence, the steering load becomes large and steering feeling is deteriorated.
For the purpose of reducing the steering load in a steering range wherein the electric motor is not driven, a technology is known which is disclosed in Japanese Patent Application Laid-Open No. 11-43062 (1999), for example. This electric power steering device is configured that the worm connected to the driving shaft of the electric motor is born by two roller bearings spaced in the axial direction of the worm so as to be movable in the axial direction.
However, in this conventional electric power steering device, belleville springs are respectively provided between the inner rings of the two roller bearings and the worm, and the movement of the worm, both ways in the axial direction, is suppressed by the elastic restoration forces of the two belleville springs.
In such kind of conventional electric power steering device, in the case where the steering force of the steering wheel is transmitted from the worm wheel to the worm while the steering is performed in a range wherein the electric motor is not driven, the worm is moved in the axial direction by the component force applied to the worm in the axial direction, against the elastic restoration force of the belleville spring. Hence, the rotational angle of the worm becomes small, and the transmission from the worm to the driving shaft of the electric motor is relieved.
However, in the configuration of the electric power steering device disclosed in Japanese Patent Application Laid-Open No. 11-43062 (1999) described above, in the case where the force applied to the worm in the axial direction is relatively large while the steering is performed in the range wherein the electric motor is not driven, the forces applied to the belleville springs may exceed the deflection limits thereof. In such a case, the belleville springs may undergo plastic deformation and become nonfunctional, or the lowering of the durability thereof may be caused.
For the purpose of preventing the plastic deformation of the belleville springs or the lowering of the durability thereof described above, it is conceivable that limiting members for limiting the deflection amounts of the belleville springs are provided between the inner rings of the roller bearings and the worm. By the provision of such limiting members, before the deflection limits of the belleville springs are exceeded, the limiting members make contact with the inner rings or the worm and prevent the belleville springs from deflecting further, thereby limiting the deflection amounts of the belleville springs.
However, the belleville springs are wholly formed into a taper shape from the inner fringe to the outer fringe so as to have flexibility. Hence, in the case where the above-mentioned limiting members are provided, each limiting member is disposed inside the inner fringe or outside the outer fringe of the belleville spring. In the case where the limiting member is disposed inside the inner fringe of the belleville spring, it is necessary to increase the diameter of the belleville spring so that the limiting member can pass through the inside of the inner fringe of the belleville spring. On the other hand, in the case where the limiting member is disposed outside the outer fringe of the belleville spring, it is necessary to increase the diameter of the worm or the size of the housing incorporating the worm so that the limiting member can be disposed. Hence, in the case where the limiting members for the belleville springs are provided, it is inevitable that the size of the worm portion is increased in comparison with the case where the limiting members for the belleville springs are not provided.
Furthermore, Japanese Patent Application Laid-Open No. 11-43062 (1999) described above discloses a configuration wherein O rings are provided on the inner circumferential sides of the inner rings and on the outer circumferential sides of the outer rings of the roller bearings for bearing the worm. With such kind of configuration, the meshing noise at the meshing portion can be reduced by using the flexibility of the O rings owing to the meshing reaction forces applied to the meshing portion of the worm and the worm wheel. However, this kind of configuration causes a problem of increasing the backlash amount at the meshing portion owing to the abrasion of the teeth of the worm and the worm wheel.
Moreover, since the belleville springs are formed into a taper shape from the inner fringe to the outer fringe as described above, if the belleville spring is assembled in such a way that the direction of the taper is reversed, the function of the belleville spring is lowered or the belleville spring does not function at all. In such a case, it becomes necessary to reassemble the belleville spring.
Still further, the difference in the dimensions of the inner fringe and the outer fringe of the belleville spring is relatively large; hence, this also causes the problem of inevitably increasing the size of the worm portion.